Near field sensor with cantilever and tip containing optical path for an evanescent wave

ABSTRACT

The disclosed optical device includes a probe which is integrally supported on a cantilever which defines an optical path extending to an external device which may carry an optical processor. 
     In a preferred embodiment the device has an optical path which is capable of guiding an evanescent wave to the external device. The device overcomes a problem with prior art Scanning Tunnelling Microscopes in that, by utilizing evanescent waves, it enables access to the so called mesoscopic range between approximately 1×10 -10  m←50×10 -10  m (1Å-50Å). In an alternative embodiment the invention permits manipulation of microscopic particles by utilising the probe. The probe may also act as a sensor.

This application claims benefit of international application PCT/GB94/01586 filed Jul. 22, 1994.

This Invention relates to optical equipment and more particularly,though not exclusively, to optical devices and equipment with integratedphotodetection means and/or processing means; or devices or equipmenthaving a direct link to processing means, hereinafter referred to as anintelligent optical sensor.

Near field Scanning Tunneling Microscopy (STM) and related techniqueshave been proposed during the last ten years to image material surfacesat a scale below the traditional, optical microscopy range, that is at,or around, the atomic scale.

Scanning Near Field Microscopy has become well established in the tenyears since the invention of Scanning Tunneling Microscopes (STM) by theNobel prize winners Binnig and Rohrer (1984). The basic idea was tocontrol the position of a probe (or antenna) in the vicinity of asurface, using high precision piezo-electric micro-actuators andfeedback electronic control of a tunnel electron current between thesurface under inspection and the probe (or antenna). More generally aso-called "proximity function" was used to control the distance betweenthe probe and the surface, as with Atomic Force Microscopy (AFM) andrelated techniques.

Scanning Tunneling Microscopes have made it possible to obtainmicroscope images of very high resolution, without using eitherdestructive electrons or a vacuum. More recently it has become possible,with computer assistance, to manipulate individual atoms in order tobuild tiny objects. The impetus is therefore now on micro-fabricationand corresponding instruments able to operate on such a micro scale.

Scanning Tunneling Microscopes (STM) and related techniques suffer fromtheir extreme specification. Thus although they are very well adapted tothe atomic scale, they are not easily extended to larger dimensionsbecause of the limited size of the so called critical distance d_(c) ofthe proximity function. Typically the critical distance, d_(c), is inthe range 0.1 nm<d_(c) <5 nm and it is within this range that STM's areintended to operate. Mesoscopic objects, which are typically of theorder of tens of nanometers (hundreds of Å), cannot easily be observedby STM, especially if the surface to be inspected is atomically rough.More recently a new technique has been proposed which relies on the socalled photon near field proximity function instead of the electronicone. This technique is fairly easily adapted to the mesoscopic domain(10 nm<d_(c) <250 nm) because of a larger value of d_(c).

Even more recently a particular technique has arisen which is calledPhoton Scanning Tunneling Microscopy (PSTM) or Scanning Near FieldOptical Microscopy (SNOM). These two techniques, PSTM and SNOM, arerelated by the ability of very small apex optical systems to receive, orto emit, photons in the near field. For the sake of simplicity thepresent invention will be described with reference to the case of PSTM,given that SNOM situation operates in a substantially reciprocal manner,it will be appreciated that the invention is also applicable to SNOM.

The main difference between classical STM and PSTM relies on the muchextended proximity function of PSTM, which makes it possible to controlthe position of a probe in an approximate range of 10 to 200 nm insteadof 0.1 nm to 5 nm for STM. This is an advantage if larger scaleinvestigations are intended on a micron scale.

It is emphasised that very small objects, that is smaller than about 0.5μm in size, cannot be seen using classical optical means of microscopy,nor can they be studied conveniently by electron microscopy. Thus makinguse of, observing and/or manipulating such small objects, has beenextremely difficult, if not impossible. Also because of the relativelylarge "size" of a photon it was very difficult to establish a specificrelationship with small light emitters such as nanometric lasers orluminescent molecules. Similarly the problem is experienced withlocalised receivers.

Thus exploration, monitoring, observation, manipulation and/ormeasurement at the nanoscale, whilst maintaining a relationship with theexternal, macroscopic world requires adapted, automated, intelligenttools which can compensate for the deficiency of human vision anddexterity at the nanoscale.

Some types of optical scanners which could be considered as intelligentoptical sensors are already in existence. One such device is describedin European Patent Application EP-A1-0509716 (CANON) and is an AtomicForce Microscope (AFM). An AFM detects an atomic force between aspecimen and a probe which is in the region of about 1 nm of the surfaceof a specimen. The device measures the amount of displacement of acantilever, which supports an optical probe, and utilises an integratedoptical circuit to control the position of the probe. The device detectsa change in luminous intensity and uses this to calculate displacementin one direction. The arrangement involves several mirrors andreflections of light as well as gratings, to split and combine lightbeams.

U.S. Patent U.S.-A1-5138159 describes an arrangement which automaticallycontrols the distance between a probe of a Scanning Tunneling Microscope(STM) and the surface of a specimen which is being observed.

European Patent Application EP-A1-0403766 describes an arrangement whichattempts to remove measurement errors arising as a result of temperaturedrift and vibrations.

The nanoscopic world is rich in applications, with a large emphasis onphoton contribution. Studies have been made of electronic"nanocircuits", which has lead to studying individual electrons andrelated electromagnetic waves. Quantum Electro Dynamics (QED) isoperated with new hybrid devices. These are hybrids of classicalelectronic and optic technologies such as interferometers and electronturnstiles. Another area is Macromolecular synthesis, which also allowscontrolled molecular structures to be built which can be used asindividual circuits, resonators or quantum wells. Nanoscopic deviceshave applications in the study of micro-biology within intra cellularstudies. This enables "in vivo" microscopy and microfabrication ofcellular matter. An important area is the study of DNA.

German Offenlegungschrift DE-A1-4231426 describes a device formed from aGallium Arsenide (Ga As) wafer or slab so as to form an optical sensor.The device is a microscopic device which cannot operate at the reducedscale of an Atomic Force Microscope.

In all the above mentioned areas it becomes very important to havesuitably adapted technical means or tools and it was with this and otherproblems in mind that the present invention arose.

It is an object of the present invention to provide a sensor withnanometric resolution which is simpler than the devices described aboveand which avoids limitations of the devices described above. None of theaforementioned documents describes a system capable of achieving this.

It is another object of the present invention to provide an opticalsensor, for inspection or profiling of materials, especially those witha high refractive indices, and in particular semiconductor materials, toa resolution of between 1 nm to approximately 200 nm with a highdetection yield and an excellent noise immunity.

It is a further object of the present invention to provide an opticalsensor for achieving an optical connection between nanometric-sizeobjects, such that the optical sensor has a sharp lateral resolution, ahigh collection yield, a high optical transfer and detection yield, andexcellent noise immunity.

According to the present invention there is provided an intelligentoptical sensor, including a cantilever supporting a probe, the probe andcantilever having an optical path, at least part of which path isadapted to transmit an evanescent wave, the path extending between a tipof the probe to an external connection of the sensor.

Preferably the probe is optically coupled to the cantilever.

According to another aspect of the present invention there is providedan intelligent optical sensor including a cantilever supporting a probe,the probe and cantilever having an optical path, at least part of thepath being adapted to convey an optical wave, the path extending to atip of the probe and to an external connection of the sensor.

Preferably the tip of the probe is pyramidal in shape. That is the tipmay comprise a square or triangular-based pyramid, which isadvantageously integrally formed with the probe, by which is meant thatthe probe and tip are formed from the same material. Most preferably thetip is of square-based pyramidal form.

Alternatively, or additionally, at least part of the optical path mayconvey an optical wave. However, the cantilever may be of semiconductormaterial transparent at the evanescent wavelength.

One or more external connections may include an optoelectronictransducer. The optical sensor may include an integrated circuit tocontrol light within the optical path. Light may be emitted and/orreceived at said tip and travel over at least a portion of said opticalpath.

According to another aspect of the present invention there is providedan intelligent optical sensor including a cantilever and a probesupported thereon, the probe having a probe tip, which probe and probetip have a first optical path, the cantilever providing a second opticalpath, arranged in optical communication with the first optical path, thedevice further including a control element to exercise control of probetip optical activity via at least one of said optical paths, and afurther optical path between said tip and external connection meansexternal to said further path, light being able to pass in at least onedirection along each said path.

The optical sensor may be adapted to permit light to pass in two or moredifferent directions along the path. There may be an optical coupling tothe probe from an external optical apparatus. This coupling may providethe external connection.

The sensor may be operable to perform at least one of forming a PSTMimage, forming an SNOM image, delivering photons to a specific site andreceiving photons from a specific site.

The sensor may operate as a sensor, a scanner or a micro-robot, or anycombination of these.

Preferably the intelligent optical sensor has a lateral precision ofimaging, or illuminating, objects in the order of 10 nm to 250 nm.Preferably the optical characteristic is the intensity of light passingalong the optical path. However, it may be the polarisation and/or phaseof the light and/or wavelength of the light. The cantilever, supportingthe probe, and tip may be fabricated using conventional micro-electronictechniques.

The optical sensor may be connected to one or more actuators. The oreach actuator may comprise a piezoelectric device.

The present invention provides an intelligent optical sensor capable ofperforming at least the following tasks: observing the nanoscopicenvironment; recognizing relief at a nanoscopic scale; and identifyingmaterials and structures, such as ridges, grooves, mesas or the like.Furthermore the optical sensor is able to bring an optical conductor,defining an optical path, to a position; and/or to convey a light beamexactly to, or from, a predetermined position.

The intelligent optical sensor, for example when used as a scanner, isadvantageously fabricated using conventional micro-electronictechniques. It can integrate several important functions on the samesubstrate by using compact integrated or hybride structures. Theseinclude:

(a) a semiconductor to make an AFM or a PSTM distance control, thelatter using a near infra red (NIR) wavelength illumination λ₁ ; and/or

(b) an optical sensor located in the immediate vicinity of the tip andtuned to a first wavelength λ₁ ; and/or

(c) an electronic amplifier and signal processor which may be digital;and/or

(d) a data transfer link for providing a link to artificial intelligenceexternal to the sensor; and/or

(e) a wavelength decoupled optical connection allowing an opticaltransfer at a wavelength λ₂ decoupled from λ₁.

An optical sensor preferably comprises a cantilever formed from asemiconductor material transparent at least at the evanescentwavelength. Alternatively the optical sensor has a probe which is formedfrom a transparent semiconductor material.

Preferably an optical sensor receives light which is emitted and/orreceived at said tip and travels over at least part of said path.

Preferably an optical sensor is arranged to guide photons between saidtip and said external connection.

Preferably an optical sensor has an external connection and includes anoptoelectronic transducer. Alternatively an optical sensor includes anoptoelectronic integrated circuit for light in said path. The opticalsensor may have a selective circuit which is tuned to a specificwavelength. The optical sensor may comprise means for supporting lightof more than one wavelength. The means may be arranged to discriminatebetween said wavelengths.

The optical sensor may comprise respective path for each of saidwavelengths.

Additionally means may be provided which comprises an integrated opticaldetector and/or amplifier, and/or an analog to digital converter and/ora digital signal processor.

An optical sensor including means to control the position of said tipwith respect to a surface of interest, in the near field region byresponse, in operation, to an evanescent wave in said optical path.

An optical sensor may include means to control the position of said tipwith respect to a surface of interest in the near field region formed atleast in part by an integrated optical detector and amplifier, ananalogue-to-digital converter and a digital signal processor.

An optical sensor may comprise means to control the position of its tipin response to a recorded atomic force. Alternatively means to controlthe position of said tip is responsive to a photon tunnelling image atthe evanescent wavelength to control tip position to a predeterminedposition.

An optical sensor as described, in which said means to control theposition of said tip is effective to permit the delivery and/ordetection of an optical signal at a predetermined position and time.

The sensor preferably has a lateral precision of imaging, orilluminating, objects in the order of 10 nm to 250 nm and may also beresponsive to the intensity of light passing along the or each opticalpath. Means may be provided to enable the sensor to be responsive to thepolarisation and/or phase of light and/or wavelength of light. Thesensor may include at least one actuator, which may comprise apiezoelectric device.

Embodiments of the present invention are now described with generalreference to the Figures and with particular reference to FIGS. 9 to 18,in which:

FIG. 1 is a sketch illustrating the basic principle of diffraction by asub-wavelength hole;

FIG. 2a and 2b show the basic principle of Photon Scanning TunnelingMicroscopy (PSTM);

FIG. 3 is a graph showing transmitted intensity of an evanescent wave(parallel polarisation) in a glass like system (n=1.5) as a function ofthe distance (d) from the interface surface and the incidence angle (θ);

FIG. 4 is a graph showing transmitted intensity of an evanescent wavewith parallel polarisation in a semiconductor system, (n=3.5) as afunction of the distance (d) from the interface surface and theincidence angle (θ);

FIG. 5 is a diagrammatical view of a PSTM arrangement illustrating keyfeatures;

FIG. 6 is a diagrammatical section of a probe of a PSTM optical fibredevice, acting as a sensor;

FIG. 7 shows schematic views of an optical path in a PSTM integratedconverter using: (A) direct coupling: (B) an inverted photodiode;

FIG. 8 is a schematic view of an optical connection in parallel with aPSTM optical control;

FIG. 9 is a diagrammatical view of a cantilever structure for remoteoptical micro-connection as part of an intelligent optical sensor of thepresent invention;

FIG. 10 is a schematic view showing how optical decoupling fordemultiplexing can be achieved with an evanescent coupling between lightguides;

FIG. 11 shows a perspective diagrammatical view of an alternativeembodiment of the sensor;

FIG. 12 shows a section through the alternative embodiment, shown inFIG. 11, in proximity with a surface of a specimen;

FIG. 13 is a perspective view showing diagrammatically intersectingplanes of interest;

FIG. 14 is a simplified diagram, illustrating a two-fold optical pathusing the two arms of the cantilever;

FIG. 15 is a diagrammatical perspective view of a bidirectionalfour-fold optical coupler for incorporating integrally with the sensor;

FIG. 16 shows a perspective view of the coupler of FIG. 15 connected toa Digital Signal Processor (DSP); and

FIG. 17 is a diagrammatical view of an additional, optional channel,suitable for use with the sensor of FIGS. 15 and 16;

FIG. 18 is a semi-logarithmic graph of intensity (I) against distance(d) of a three medium refraction model, regions -a- and -b-corresponding to different angles of incidence;

FIG. 19 is a diagrammatical sketch of an experimental arrangementincorporating an alternative embodiment of the invention;

FIG. 20 is a graph of extrapolated exponential shift (To), against angleincidence (θ) for s and p polarisations, wherein n-n'=1.5, b) n=1.517;n'=-2.1;

FIG. 21 is a semi-logarithmic graph of intensity (I) transmitted byprobe tip, against distance (d);

FIG. 22 is a diagrammatical sketch showing light capture and guidance bythe cantilever; and

FIG. 23 is a logarithmic polar plot for points P1 and P2 in which A(d˜0)and B(d=200 nm) for a variety of polarisations.

FIG. 1 shows as a model and is useful in considering an object having acharacteristic dimension less than illuminating wavelength λ_(a), Inthis situation the Maxwell equations are applied in order to calculatecharacteristics of scattered waves. Dual situations of a smallscattering particle, or a small circular hole in a shield, give rise todual conclusions. A shield 10, in which a hole 11 is defined, thecircular hole 11 having a nanometric apex, is illuminated by apropagating plane wave with wavelength λ_(a). The details of the shapeof the hole 11 are transmitted to the scattered wave as long as thedistance from the hole 11 (or object), is in the same range as its size.This is known as the near field and occurs close to the hole in thevicinity 12A. This distance is typically within a region less than thewavelength. At greater distances, the propagating wave becomes sphericaland does not convey any information as to the shape of the hole 11.Region 12B is the "far field".

The hole 11 can be employed as a structuring element and can effectivelybe used for scanning imaging. The hole 11 can be used either as a pointlight source as shown in FIG. 1 or as a point receiver (not shown) forexploring a light field scattered by a physical object (not shown). Theformer simplified situation corresponds to SNOM, the latter to PSTM. Inreality both SNOM and PSTM require the same condition that the hole 11has to be at a very short distance from the specimen under inspection.

For the sake of simplicity only the case of PSTM is considered below, itbeing understood that the same conclusions and principles can beincorporated in SNOM.

The basic principle of PSTM is shown in FIG. 2a. A light beam 13 isshown propagating in an homogenous dielectric material 14. The beam 13is reflected at an interface with a second, different, material 15having a lower refractive index than material 14, if the angle ofincidence of beam 13 is greater than the critical angle of refraction atthe interface. Such total internal reflection generates in the secondmaterial 15, an evanescent wave 16 which passes along the interface ofthe two materials 14 and 15 with an amplitude (I) which rapidlydecreases with the distance (d) to the interface. The relationshipbetween intensity, distance and polarisation angle of incidence light isshown in more detail in the graphs in FIGS. 3 and 4. The criticaldistance d_(c) of the exponential function, which governs this rapiddecrease in amplitude is found to lie in the range 10-300 nm. It isunderstood that this distance depends on the angle of incidence and therefractive indices of the materials. Therefore by selective variation ofthese variables the critical distance (d_(c)) may be varied within agiven range as depicted by the graph in FIGS. 3 and 4.

Usually the second material 15 is simply air whose refractive index is 1(n=1). If a third dielectric material is placed in the near field regionof evanescent wave 16, electrons from the third material can couple withthe electromagnetic wave and give rise to a propagating mode. Atransmitted light beam is then created in the third medium, through theair gap. The situation is similar to the electron tunneling effectthrough a potential barrier and is depicted diagrammatically in FIG. 2b.

FIG. 2(b) shows a dielectric probe 17, for example an optical fibre,which can capture an evanescent wave (not shown) from a sample material141, illuminated as described above. Capturing the evanescent wavegenerates a photon flux in the probe 17. The smaller the gap d(x),between the tip of probe 17 and the surface of the material 141, thestronger is the collection yield. This phenomenon is called FrustratedTotal Internal Reflection (FTIR) because energy in a transmitted lightbeam is deducted from energy in the reflected beam.

Optical probe 17 can be scanned over the surface of the sample materialin the x-direction of material 141. Probe 17 may also be moved inperpendicular directions to follow relief of the surface. Control andfeedback means (not shown) may be used to maintain a constant photonflux in the optical probe 17. Monitoring the photon flux and maintainingthis photon flux at a constant, enables a profile of the surface to beobtained.

PSTM can be considered from many points of view as the photonicequivalent of STM. There are two important measurements which can beobtained. The first is measurement of information pertaining to thetopography of the surface of the sample material at a nanoscale. Thesecond is that information pertaining to the chemical nature of thematerial which is locally related to the critical distance (d_(c)) andto the refractive index (n).

It has been reported that the lateral resolution and precision of PSTMis primarily affected by the dimensions of the apex 171 of the opticalprobe 17, as shown in FIG. 6. The apex 171 protrudes beyond tip 172 ofprobe 17. The distance d(x) the tip 172 has to be placed, with respectto a surface of a sample material (not shown), must be in a similarrange to the distance the apex 171 protrudes beyond the tip 172. It hasbeen shown that the exact shape of the tip 172 has little effect on thecapture yield. Thus it is possible for the purpose of calculations, toconsider the tip 172 as being flat. Using this approximation thetransmitted intensity of coupled photon flux has been calculated forPSTM glass systems and also for semiconductor systems in the near infrared (NIR) region of transparency. The latter material is a special casebecause of its relatively high refractive index.

Examples are given in the graphs shown in FIGS. 3 and 4 respectively forn=1.5 (glass sample and tip) and for n=3.5 (semiconductor sample andtip) as a function of the distance d (x) and the incidence angle (θ) fora wavelength of 1μm and a polarisation parallel to the incidence plane.The intensity (amplitude) is in arbitrary units. It is observed that forthe semiconductor case, which is shown in FIG. 4, that a much shortercritical distance (d_(c)) (seen by the steeper slopes in FIG. 4) in therange of 10 nm or less is achievable. This allows higher resolution andmore precise control of the tip 172. It may thus be concluded that PSTMis a technique which is particularly well adapted to semiconductoranalysis, provided that the probe 17 (or antenna as it is sometimescalled) is also made with a semiconductor material (or similar material)and has a high refractive index (n), so as to promote efficient opticalcoupling. Details of such an arrangement are described below withreference to FIGS. 7a and 7b.

Most PSTM investigations have been devoted to glass or biological likematerials (n=1.5). A typical (glass) optical fibre is a very convenientconverter. A general view of an experiment is shown in FIG. 5.

Referring to FIG. 5, there is shown a prism 18 in optical connectionwith a sample material 19. An air gap 21 separates sample material 19from probe 22. A path of a wave λ is shown by the arrow connectingpoints A and B. The intensity recorded in the probe is related to thedistance d. Refractive indices n₁, n₂, n₃ and n₄, of materials 18, 19,21 and 22, respectively are indicated as is and the orientation θ₁ ofthe beam incident. Usually the probe 17 is an optical fibre in which n₄₌1.5.

An optical fibre 23 is shown in greater detail in FIG. 6 in which likeparts bear the same reference numerals as in previous Figures. Opticalfibre 23 is tapered by thermal stretching or by chemical etching and itis cladded by a metal coating 24 evaporated under vacuum. An apex 171 isformed by drawing the fibre 23 so that a neck 171, some 20 to 100 nmwide, is produced. The metal coating 24 has also been used to combine anelectronic STM operation simultaneously with the PSTM.

The use of a semiconductor, such as silicon nitride (SiN) tip as a probeto make PSTM measurements is described by N F van Hulst et al. in SPIEConference Los Angeles, 1992, Vol. 1639 pp 36. An experiment isdescribed as using an AFM device and a SiN probe (n=2) to detect an FTIRsignal. This experiment effectively combined PSTM measurement with AFMoperation.

The techniques described above, representing specific aspects of theinvention, show two of the advantages of the invention. The first isthat an arrangement of semiconductor tips and integrated detectionproduces more reliable imaging of surface relief than hitherto possibleand thus makes possible the intelligent positioning of a tip at adesired position on the surface of a sample. The second is that theoptical guidance properties of the tip permit the parallel passage of asecond light beam, (having wavelength λ₂ different to a first light beamof wavelengths λ₁), between an external system and a position on asurface of interest. The position on the surface may be specified withnanometric precision. The light beam can pass in either or bothdirections.

It will now be explained how the invention can be used in technicalapplications and in particular in the field of nano OEIC technology.Some preferred embodiments are described with reference to Figures.

One embodiment is now described with reference to FIGS. 5, 7a and 7b. Inorder to provide satisfactory resolution and controllability in PSTM, itis necessary to match the optical indices of prism 18, sample 19 andprobe 22, as shown above in FIG. 5. This means that dealing withsemiconductor samples requires the use of a semiconductor probe. Thiscan be achieved in a configuration similar to that of AFM tips whichalready use doped silicon or silicon nitride (SiN) as a transmissionmaterial. This particular solution permits optional combination of theprobe 22, with an optical guide, a photodiode, an amplifier and even adigital signal processor (DSP) on a single chip or in a compact hybridstructure.

In order to provide a means for convenient collection of photons asemiconductor probe can be arranged with an internal refractive indexgradient similar to that of an optical fibre's graded index. Chemical orplasma etching enables the fabrication of semiconductor pyramidal orconical tips or carbon "super" tips with a sharp end of aperture size inthe order of 10 nm.

Referring to FIGS. 7a and 7b, an optical coupling 222 for a tip 221 isconnected either directly (FIG. 7a) or via a substrate 224 (FIG. 7b) toa photodiode 225. Due to the very high refractive index ofsemiconductors, such probe tips 221 benefit from a very high collectionyield and also a very good confinement and guiding ability for photonsin the visible or near infra red region (NIR). These aspects may beimproved by the arrangement of an internal gradient index structure. Theprobe tip 221 can be directly tailored in the substrate 224 material(e.g. silicon or a similar material) or grown in layers of suitablyadapted refractive indices. Probe tips 221 can also be provided with ametal cladding (not shown) to prevent stray light capture/loss. Theassociation of the probe tip 221, the optical guide or coupling 222, thephotodiode 225 and signal processor (not shown) on the same chip, invery close proximity to one another, guarantees an exceptionalconversion yield of the sensor and very good noise and dark currentimmunity. Materials are transparent to the light to be passed. FIG. 7ashows a tip directly grown on a photodiode 225. FIG. 7b shows a tiparranged remotely from a photodiode 225, which is in an invertedposition. Substrate 224 may be semiconductor chip material and may form,or be part of, an optical cantilever as shown in FIG. 9 and describedbelow. A thin film filter 223 is associated with the system to providewavelength selection.

FIG. 8 shows a more complex OEIC structure which is arranged as abi-directional optical connector 283. Like parts in FIG. 8 bear the samereference numerals as FIGS. 7a and 7b. Double headed arrows indicateoptical connectors, whereas single headed arrows show PSTM controllinks. The connector 283 ensures that light follow the same path in theprobe tip as does the PSTM control beam 222. This is schematically shownin FIG. 8 where double headed arrows denote bi-directional opticalconnection paths. References 281 and 282 denote respectivelyoptoelectronic transducers for internal and PSTM optical signals. Timingor other forms of discrimination can be provided for light at anywavelength (λ₁, λ₂, etc.). Also different paths may be allocated fordifferent wavelengths of light.

Additionally, or alternatively, one or more input(s) and/or output(s)to/from optical beam 302 can be provided by an external coupling 301.The external coupling 301 may be achieved for instance with a lens or anoptical fibre (not shown). Light may also be introduced by internalgeneration (LED or semiconductor laser) of the particular wavelength(s)desired. FIG. 8 shows a function of nanoscopic imaging which the PSTMprobe of the sensor also allows. This is the positioning and control ofthe probe tip 221 in such a manner that optical transfer is achievedwith the sample. It is also convenient that at the same time probe tip221 yields other information, for instance by means of measurement ofatomic or lateral forces or electron tunnel current. Optical transferwith a sample may be in the form of a signal path, which may be both toand from the sample. For example an array of microlasers may be testedby establishing a one-way or two-way signal path for optical transferwith a specific microlaser.

A further embodiment is shown in FIG. 9 which shows a remote detectionsensor for collection of photons. Collection of photons is provided by adevice similar to AFM devices. A probe tip 30 is carried by a side arm(or cantilever) 31 which acts as an optical guide. The side arm orcantilever 31 may also incorporate a beam splitter (not shown) to directphotons to/from opto-electronic integrated circuit (OEIC) 32. Asemiconductor substrate 33 couples photons to and from the OEIC 32. Thearrangement may be connected to one or more piezoelectric actuators (notshown). An external optical connection 34 is also provided. This maylink the device to an optical fibre, a lens or a microscope (not shown).

It is important to select different optical beams at differentwavelengths and to direct them to/from their respective circuit. Thisoptical demultiplexing can be achieved classically either by a beamsplitter associated with parallel filtered detectors, as shown in FIG.8, or by evanescent light coupling between integrated guides as showndiagrammatically in FIG. 10.

The structures described above can be fabricated by followingconventional semiconductor techniques. The sensor is devoted to severalsimultaneous tasks: firstly PSTM imaging of a surface or sub-surface;secondly PSTM control of the three dimensional positions of the probetip at a predetermined location; thirdly simultaneous AFM or STM or anyother local measurement; and fourthly collection or delivery of photonsat a specified place. It will be appreciated that not all of these tasksmay be carried out in a particular device.

An intelligent optical sensor, eg a scanner, is fabricated usingconventional micro-electronic techniques. It integrates severalfunctions on the same substrate by using integrated or compact hybridestructures. Structures may comprise a semiconductor to make an AFM or aPSTM distance controller, the latter using a near infra red wavelengthillumination λ₁ ; and/or an optical sensor located in the immediatevicinity of the probe tip and tuned to the wavelength λ₁ ; and/or anelectronic amplifier and signal processor which may be digital; and/or adata transfer link for providing the possibility of artificialintelligence; and/or a parallel and wavelength decoupled optical lead,allowing an optical transfer at a wavelength λ₂ decoupled from adifferent wavelength λ₁.

One embodiment relates to an optical device for telecommunications andoptical computers. Optical systems are especially well adapted to theparallel nature of data processing involved in telecommunication devicesor optical computers. Such matrix organisations of surface devicesrequire small but numerous identical nanometric components such asmodulators, bistable resonators or surface emitting lasers. These areintended to be key devices for signal communication (asynchronous timemultiplexing) or programmable arrays of optical memories.

An aim of the present invention is to provide a means, including but notlimited to method and apparatus, for inspecting and for individuallytesting the components in position and/or programming such complexsystems of surface nanodevices.

A second embodiment relates to the replication of surface devices on alarge scale of integration where it is mandatory to satisfy goodreproducibility of morphologies. For instance, surface emitting lasersrequire a precise patterning in order to achieve a rejection of lateralmodes. Nanolithography usually performed by electron beam X-rays orfocused ion beam (FIB) require precise control, as do the transfertechnologies such as lift-off, reactive ion beam (RIE), localisedimplantation and diffusion. For instance quantum wires are known to beaffected by residual resin molecules which induce limit fluctuations inthe dimensions.

A further aim of the present invention is to contribute to theprevention of such defects by providing means, including but not limitedto method and apparatus, for optical detection and local operation.

A third embodiment relates to new mesoscopic devices which have recentlybeen developed such as mesoscopic metal rings, interferometers,Josephson diodes, Coulomb blockade structures, single electrontransistors or even magnetic SQUIDs, or microstructures using high T_(c)(Curie temperature) conductors. All these require micromachining,microcontrol and more essentially a microconnection (e.g. microcavityoptical resonators) for their relation with the external world.

A yet further aim of the present invention is to provide means,including but not limited to method and apparatus, for new experimentalphysics at a nanoscale of quantum electronics.

Such a complex arrangement has to be automated as far as possible.Corresponding signal and data processing has to be designed andspecialist software programmed for each case, as will be appreciated bya skilled person. This may require remote computer processing, even ifsome low level control and feedback is being operated on specifiedDigital Signal Processing (DSP) integrated on the chip. Such signalprocessing can be obtained from the usual computer techniques of imageprocessing, data fusion, fuzzy logic so as to make the sensor expert and"intelligent".

The sensors described above are able to collect (or emit) photons with avery high spatial precision in order to bring about a high resolutionoptical connection. The sensor uses an Atomic Force Microscope (AFM)standard tip to guide photons to or from a precise location on a surfaceto or from an optoelectronic detector or emitter. The detector oremitter may be equipped with associated electronic circuitry dedicatedto signal processing such as amplification, digitization, digitalprocessing, or other types of artificial intelligence. Brief referencewill now be made to FIGS. 11 to 17 and to various manufacturingtechniques of the sensors.

Standard AFM tips currently fall into two categories. Firstly siliconoxinitride (SiN) pyramids which are obtained by Organo-Metallic ChemicalVapour Deposition (OMCVD) into a Silicon mould. This is depicteddiagrammatically in FIG. 11. Secondly Silicon tips have very sharpfeatures which may be obtained by chemical or plasma erosion.

Contrary to previous opinion, SiN walls of pyramidal tips used in AFMare able to convert an evanescent wave at its surface into a propagatingmode of photons which conserves the photon's momentum direction. Thisparticular property makes it possible to selectively guide photonstowards or away from different optical detectors by means of differentoptical paths. One such example in the path corresponding to differentarms of a cantilever 40 which supports a tip 42 in the form of apyramid. The intelligent optical scanner may then be split into amultichannel bidirectional sensor, namely a double channel system, maybe easily organized through the two arms using standard tips. An exampleis shown diagrammatically in FIG. 12.

FIG. 11 shows general features of an AFM probe 110 which has been formedfrom a SiN layer. The probe 110 has a tip 112 which is placed at the endof a two arm 114, 116 cantilever 118. The cantilever 118 Is stuck to asubstrate 120 using a suitable adhesive. The substrate (not shown) maybe PYREX (Trade Mark) material.

The pyramidal shape of the probe tip 110 is obtained by the followingsteps:

i) a silicon wafer is equipped with a mask (not shown) with limitingsquare windows (typically 5μm wide). An etch is performed whichselectively reveals the 111! planes thus creating a pyramidal hollow,whose internal face angle is 57°.

ii) resin is removed and the silicon surface is covered with a SiN thinfilm layer (typically 0.8-1μm thick) where cantilever 118 is patterned.The pyramidal hollow behaves as a mould. The walls of the mould arecoated with the SiN layer.

iii) A PYREX (Trade Mark) substrate is stuck onto the external face ofthe pyramidal mould, as a sample holder and then the Silicon substrateis dissolved chemically.

In recent work the inventor has performed a Photon Tunnelling Microscopyexperiment, shown in FIG. 13, in which like parts bear the samereference numerals, using an AFM SiN tip, (fabricated according to theabove steps) as a photon converter. It was already known that such a tipwas able to convert an evanescent wave into a propagating mode emergingfrom rearface of a cantilever 118 in a perpendicular direction. Theinventor has discovered that a guided mode was also present in the tip112 walls, such that a strong emission mode was noted from one side ofthe cantilever 114. This was approximately in the initial direction ofthe beam and also through the lateral face of the cantilever 118. Thisfeature of the invention which uses the directional guiding property totransmit photons in the tip 112 selectively to one or other side arms114 or 116 of the cantilever 118, (depending on the initial direction ofthe incident photon), enables the tip 112 to behave as a double channeltransmission system. The optical transmission of each channel (or arms)114 or 116 is bidirectional.

Light emission from the tip 112 to substrate 120 can be made as well asits collection in the reverse process. Similarly such a sensor can bemade with more than two channels even if desired. The aim is to guidecollected photons dominantly towards the arms 114 and 116 of thecantilever 118. This means that the incident beams have to beconveniently directed, in the substrate 120, in such a way that theycould be transmitted into a corresponding tip wall in a convenientdirection. For a better yield of transmission and limitation of lostlight, it is advantageous to arrange the orientation of the probe tip asshown in FIGS. 13 and 14.

A possible realisation is shown in FIG. 14. In this arrangement the tipof the pyramid has been rotated through 90° in a direction horizontalplane perpendicular to the plane of the Figure, of the pyramid in orderto direct the walls 220 and 222 equally towards the right 116 and theleft 114 arms of the cantilever which will make the guiding operationeasier. In this realization both arms 114 and 116 play the role ofindependent optical guides between the tip and the optoelectronicindependent integrated circuits 140 and 142 emitters as well asdetectors.

The arrangement in FIGS. 13 and 14 makes it possible to establish anoptical connection through the tip 112 using two (or possibly more)independent bidirectional channels thus allowing multiplexing ofparallel optical links between the nanoscopic and the macroscopic world.For instance one channel may be used for tip position control, whilstthe other channel is used simultaneously to transmit information,possibly even at the same wavelength, i.e. λ₁ =λ₂.

Scanning Tunnelling Optical Microscopy (STOM) solves the problem ofobtaining information from a surface and subsurface of transparentmaterials. A light beam introduced into the material and totallyreflected internally (TIR) at the surface provides a non propagatingevanescent wave on an external side. A dielectric probe may then beplaced in the near field region, as shown in FIG. 1, and atomic couplingwith this electromagnetic field gives rise to a propagating mode in thedielectric medium of the probe. The reflection is said to be"frustrated" (FTIR). Usually the probe is simply a tapered glass opticalfibre as shown, for example in FIG. 6. More recently van Hulst et al.and others, have reported the use of Silicon Nitride (SiN) Atomic ForceMicroscopy (AFM) stylus to collect an evanescent field. Such probesconveniently perform a smooth approach to the sample surface becausethey are carried by a flexible cantilever. At the same time therelatively high refractive index (n=2.1), and the transparency of thematerial from which they are formed, provide satisfactory transmissionof light.

Commercially available pyramidal (SiN) AFM tips make it possible toperform the PSTM experiment with a spatial resolution of 50 nm or lessand to obtain AFM data during the same scan. An experiment for such asystem is described below.

Another embodiment of the invention is one having only one arm, and afour channel, bidirectional coupler 600 and is indicated generally inFIG. 15 and in greater detail in 16. Four small OEICs 602, 603, 604 and605 (light detectors or emitters, as well) are placed close to the tip606 in such a way that they are on optical paths corresponding to eachof the four walls of pyramidal tip 606. There is sufficient space toplace such devices at the end of cantilever 607 which may comprise asingle or multi arm device. In this configuration the or each arm is notused as a light guide, but its role is to carry a bundle of electricalconnections 608, 609, 610 and 611 or as well a combination of bothoptical and electrical connections.

The electrical leads from the OEICs 602, 603, 604 and 605 to thesubstrate circuit which may be a DSP or to a connection to externaldevices can be obtained using evaporated metallic or ion implantedconducting lines. Such an optical coupler benefits from four independentoptical paths allowing for example multiwavelength, simultaneous PSTM orany combination of optical connections as described above. Otherappliances which may be connected include local photoluminescent devices(excitation plus detection) and/or optical tools for biology such asimaging, optical tweezers or scissors.

Optical cross talk between channels may be limited by convenient opticalbarriers in the bulk of the material forming the or each arm. Theseinclude trenches, implantation of ions inducing absorption of photons oroptical index changes.

Optical and electrical circuits of such an intelligent sensor may becoupled with the piezoelectric self control of the flexion of thecantilever arms.

Light may be provided by one or more of the channels and can bereflected by the surface of the sample and then again captured byanother channel as sketched indicatively in FIG. 17.

The above configurations can be effective in many various situationssuch as optical near field microscopy, high density data storage,nanotechnologies, optical communications and optical computers.

The evanescent near field intensity in the free space above a flatsurface of a dielectric is traditionally described by the exponentialformulation: ##EQU1## where d is the distance to the surface and is acritical distance; λ is the wavelength, n is the refractive index and θis the internal incidence angle.

The physical situation of PSTM is more complex than suggested in Eqns. 1and 2 because of several perturbations.

The presence of a third medium for photon collection in the near fieldclose to the surface, introduces a deviation to the exponential regime.The intensity of a reconstructed propagating wave can be calculated bymeans of Maxwell equations in the case of infinite plane surfaces. Inthis case also the exponential asymptotic regime can be observed (asseen in FIG. 18) at a large distance (d>d_(c)) if detection is sensitiveenough. The initial value T_(O) may be extrapolated from a semilog plot,its value can be larger as in curve (a) or smaller curve (b) than 100%depending on n, and on the polarization as has been shown in computersimulations.

It has also been established that relief corrugation can lead to verycomplex three dimensional combinations of the evanescent field and alsoto spurious propagating modes at places where the critical incidence forTIR is not respected. To avoid such difficulties it has been assumedthat surfaces are satisfactorily flat.

The shape and the size of the probe tip has to be considered. The tip isa subwavelength object which is far from the infinite planeapproximation. Nevertheless it has been shown that the deviation ofexperimental results is generally not as dramatic as may be expected.All these restrictions being considered, the capture of the evanescentwave by a dielectric tip classically suggests two differentexplanations. Firstly the tip is considered as a plane dielectricinterface obeying the set of macroscopical Maxwell equations. Secondlythe tip is considered as a subwavelength scattering particle (Miedipolar scattering) in which isotropic emission (near and far field) ispartly collected in the apex of the tip. These different situations infact are not so clear cut and occurrence of other unknown phenomena havebeen recently invoked to explain non consistent experimental results.

The experimental arrangement shown diagrammatically in FIG. 12 involvesa semicylindrical glass prism 500 (n=1.517) of which the surface isdirectly taken as a sample under test. Light is provided by a HeNe laser502 (λ=0.633 μm and 1.5 mW) and pyramid tip 504 is standard SiNproduction from the TOPOMETRIX company.

Although the PSTM technique was initially invented to give superresolution relief mapping, it appears that very strong disturbancesoccur in the near field with relief corrugation. In the present casegreat care has been taken to verify that the surface was sufficientlyflat, given experimental circumstances.

Phase Stepping Microscopy (PSM) images, shown at VDU 506 were obtainedfrom the prism 500 surface. These show that observed relief featuresgive a height to length ratio larger than 100, which is a convenientrange to keep clear of PSTM propagating mode artefacts.

Face angles of hollow tip 504 pyramid are 55°. The thickness of a CVDSiN layer is 1 μm. Spring constant of cantilever 508 is 0.064 N/m andthe tip radius is less than 50 nm. AFM probe is adhered onto a sampleholder associated with a piezoelectric actuator, indicated by arrow A,at an angle of 15°. The rear of the tip 504 is observed with a CCDcamera 512 without an automatic gain control. The camera 512 wascarefully calibrated. Image processor unit 514 allows data to beobtained.

The incidence angle E was chosen to be λ=46° in order to be in excess ofthe FTIR limit but also to be small enough not to induce spuriousscattering from the surface and to keep a high laser intensity on thesurface. The corresponding attenuation length is d_(c=) 265 nm. Twoexperiments were performed. These are described below.

The first experiment relates to the transmitted intensity as a functionof tip/surface distance. Piezoelectric actuator allows displacement ofthe tip with respect to the prism, but the exact tip to sample distancedepends upon several uncontrolled parameters. These include:piezoelectric element hysteresis; cantilever flexion, induced by Van derWaals forces; electrostatic charges; and/or radiation force of theevanescent wave. Nevertheless it is expected that such pertubations,which could be large in the distance range of 0-50 nm, do notappreciably influence slope measurement at a distance of d>250 nm.

Each set of measurements was performed by approaching the tip to thesurface. After touching the surface of the prism 500 tip 504 sticks to awater film (not shown) and an effort is required to remove it. Theobjective (×3.2) is placed at a distance such that its aperture isobserved from the rear side of the tip 504 at an angle of less than 20°.The magnification allows an image of the base of the pyramid supportingtip 504, to be obtained (4×4 μm) over an Area Of Interest (AOI) of 3×3pixels. The intensities (256 grey levels) were integrated. Dark currentoff-set is classically obtained from a large distance plateau and isdeduced from data to give the actual intensity.

An advantage of collecting transmitted light with a remote camerainstead of an optical fibre essentially relies on long distanceoperation which prevents interferences occurring between the tip 504 anda CCD sensor. This also provides a narrower angular resolution ofobservation.

The second experiment required the camera 512 to be rotated in theincidence plane, about the tip 504, in order to keep it focussed on thecentre of the image, thus maintaining a constant distance between thetip 504 and the camera 512. The ratio of flux of the Poynting vector onthe camera sensor (0° direction) to that of the laser 502 at the prismsurface 500 has been found to be 1:10⁵. The laser 502 at the prismsurface 500 conveniently indicates a collection cross-section in a rangeof (50)² nm² at the tip 504 of the pyramid. The maximum rotation anglewhich is achievable on both sides by the camera is 60°.

The amplitude of the extrapolated exponential shift T_(O) wascalculated. In the present case using the refractive index n=1.517 inthe prism 500 which is different from that in the probe tip 504 wheren'=2.1. The calculation was modified, eventually giving the formulaexpressed in Eqn. 3 below. ##EQU2## where ρ=±1 depending on the s or ppolarisation respectively.

It is observed from the graph in FIG. 20 that the high index in theprobe appreciably changes the coefficient T_(O) for both polarizations.Some changes can be foreseen in the 1(d) curves in the region O<d<d_(c),even if longer distance logarithmic slope still remains unaffected.

In intensity dependence observation with respect to distances, thecamera 512 was placed at 0° above the tip 504. The distance d is variedand the image is recorded. Back face of the cantilever 508 shows abright point, referred to below as P1, at the tip position. The lowmagnification does not allow determination of whether the light emanatesfrom the centre of the hollow or from the side walls. It was also foundthat p polarization gives a much brighter transmission than the s one.

The data was normalized to a 100% level corresponding to the opticalcontact (d=0). They are shown in a semilog plot in FIG. 21 on anarbitrary distance scale. It was observed that the theoretical slope(d_(c) =265 nm) conveniently agrees with experimental data. The Toplevel, placed in the graph, allows the position of the d=0 point to bedetermined on the distance scale. In order to place other data fromdifferent experiments in the same graph, a similar operation wasperformed on the data and each plot was shifted laterally to bringasymptotic slopes into coincidence, thus establishing a common origin(d=0) at a fixed point on the distance scale.

Good agreement is found with computed predictions with the classicalrefraction model, macroscopical semi-infinite dielectric capture, evenif the distance scale is not exactly in the range of d>50 nm. This isdue to the above mentioned reasons of cantilever flexion. It is alsoworth observing that the s polarization should certainly not be confusedwith the actual results.

FIG. 21 also shows results obtained with the camera 512 rotated 33° and45° counterclockwise (towards the beam). A strong increase in thetransmission is noted when the camera 512 is rotated and the distancedependence is again measured. The variation of the intensity with thedistance still remains unchanged. This demonstrates that the light doesnot emanate from a stray capture, but from the FTIR operation in the tip504. A likely explanation is that it is the inner walls of the tip whichguide captured light until it reaches the outside surface of thecantilever as shown in FIG. 22.

Another observation for such positions of rotation of the camera 512 isthe appearance of a second bright point (P2) located on the edge of thecantilever in the incidence plane as indicated in FIG. 22. This sourceof light is only active for the s polarization and its intensity varieswith the distance as expected. The light source corresponds to photonswhich are guided in the cantilever 508 from the tip to the left section.

Angular dependence of transmitted light occurs when the camera 512 isrotated both sides from the 0° position. Both points P1 and P2 disappearfrom the right side (clockwise) whereas P1 becomes brighter for the leftside and the point P2 appears. The results are shown in FIG. 23 in anangle diagram. Two series of measurements are made corresponding to twodifferent tip/sample distances (A:d˜200 nm B:d˜o) and for differentpolarization conditions.

In addition to the axial emission P1 shows a large lobe in the direction40°-60° which corresponds to the wall direction (25°-35°) corrected fromthe refraction. The p polarization is preferentially transmitted overall of the angular domain. Likewise P2 does not display an axialemission but a large lobe increasingly intense at high angles. In thelatter case the p polarization is not transmitted likely due to internalreflections.

From the above observations a first conclusion is that the transmittedphotons emanate from the conversion of the evanescent waves in the tip504. The distance dependence corresponds fairly well to usualmacroscopical model of refraction. A second point is that lateraltransmission only, in the left side wall, indicates that the photonmomentum is satisfactorily conserved in the tunnel effect experienced bythe photons. It can then be deduced that the tip conversion does notinvolve a dominant point scattering mechanism, which should beisotropic. This point could not have been directly revealed with auni-directional cylindrical optical fibre.

It is also worth noting that the preferred direction of the propagatingmodes, created in the dielectric stylus, is the same as the direction ofthe radiation force experienced by subwavelength objects placed in anevanescent radiation field. This preferred direction of photon momentumis also typical of the refraction model in spite of the subwavelengthsize of the tip 504.

The intensity of the light transmitted by an SiN AFM probe used in aPSTM configuration has been observed at various angles of transmission.It has been shown that in addition to an axial lobe of intensity, astrong intensity also appears in the direction of the tip walls andselectively on the side which is orientated along the beam direction.The dependence of these intensities with the distance is in agreementwith the macroscopic model of capture of a plane evanescent wave by asemi infinite dielectric medium. The scattering contribution of the tipseems negligible in this experimental situation. It is also observedthat the propagating modes are efficiently guided by the walls of thetip to the lateral edge of the cantilever 508.

It will be appreciated that the above embodiments have been described byway of example only and variation to them is permitted within the scopeof the invention. Examples of these include arrangements described inthe following paragraphs.

These arrangements include an optical device described in the followingtwo principal paragraphs as modified by arrangements described in one ormore of the following paragraph(s).

An optical device may include a cantilever supporting a probe, the probeand cantilever having an optical path, at least part of the path tocapture an evanescent wave and extending to a tip of the probe, the pathalso extending to an external connection of the device.

An optical device may include a cantilever supporting a probe, the probeand cantilever having an optical path, at least part of the path toconvey a longitudinal wave and extend to a tip of the probe. The pathalso extends to an external connection of the device.

We claim:
 1. An optical sensor comprising:a light source of apredetermined wavelength; a probe having a tip, said tip havingdimensions which are small with respect to said predetermined wavelengthfor capturing light from the light source from a surface of interest inthe near field region of said tip, and said tip having walls for guidingthe captured light; a cantilever supporting said probe; a first detectorintegrated on a substrate, said first detector being arranged to receivethe light guided by a first wall-of said tip in the direction of saidfirst wall, and an external connection receiving light by a second wallof said tip in the direction of said second wall.
 2. An optical sensoraccording to claim 1 wherein said probe is made from a semiconductormaterial which is transparent at the predetermined wavelength.
 3. Anatomic force microscope comprising a sensor according to claim 1including means to control the position of said tip with respect to asurface of interest, said means comprising an analog to digitalconverter and a digital signal processor connected to said digitalconverter and wherein said substrate comprises an amplifier connected tosaid digital converter.
 4. A scanner comprising a sensor according toclaim 1 and means to control the position of said tip for deliveringand/or detecting an optical signal at a predetermined position and at apredetermined time.
 5. An optical sensor according to claim 1, furthercomprising an analog to digital converter and a digital signal processorconnected to said digital converter, and wherein said substratecomprises an amplifier connected to said digital converter.